Research has shown that the degree of uniformity of the distribution of ammonia in a pipeline of an exhaust gas post-treatment system (e.g. a selective catalytic reduction system, SCR system) has an important influence on the overall performance and durability of the system. If the distribution of ammonia is not uniform, this will lead to an excessive amount of ammonia in local regions and thereby easily cause leakage of ammonia, and cause an excessively low efficiency of conversion of nitrogen oxides (NOx) in other regions where ammonia is rarefied. A non-uniform distribution of ammonia for a long period of time will lead to non-uniform ageing of the catalyst, thereby affecting the overall performance of the catalyst. Furthermore, a non-uniform distribution of urea droplets will cause an excessively low temperature of a local pipe wall or mixing structure, and crystals will form; in serious cases, the exhaust pipe will be blocked, leading to a drop in the motive power performance of the engine.
Chinese invention patent no. CN205714382U, announced on 23 Nov. 2016, has disclosed an engine exhaust gas post-treatment system in which a multi-holed pipe is added in a mixing pipe. Such a solution reduces the probability of urea coming into direct contact with a pipe wall, because the urea can be broken up in advance, and reduces the risk of urea crystallization to a certain extent. However, in that design, due to the fact that no multi-holed pipe is disposed at the position of a fin, urea droplets can directly strike the fin as well as a junction between the multi-holed pipe and the mixing pipe, so the risk of crystallization remains. Furthermore, the question of how to better support the multi-holed pipe is also a technical problem in need of a solution.